In plastic part production it is known to produce parts in large batch sizes and series by injection moulding or extrusion. The advantage of plastic injection moulding in particular rests in particular on the highly precise production of complex part geometries, wherein the versatility of operation of the injection moulding process covers the requirements for an inexpensive and economical production of plastic parts in an optimal manner.
At the same time, there is an ever-increasing demand for plastic parts in a unit number of one and small batch sizes such as e.g. prototypes with the requirement of provision in a very short period as well as properties that are similar to those of injection moulded parts. For the production of such parts there are production processes that are widely known by the term prototyping. The production of such parts, in most cases, is based on generation of the geometry from 3D data. These geometries are produced in a wide variety of forms by appropriate means such as melting powder layers by the application of heat, e.g. using lasers, additive systems such as compression processes in a different connecting form of the powder parts or also in the so-called melt extrusion process.
A device, wherein a plasticising unit known in injection moulding technology is coupled to a pressurisable material storage device for the liquid phase of a material, is known from DE 10 2009 030 099 B4 that forms the basis of the preamble of claim 1. To generate an object on an object support in a construction area, this material is discharged in the form of drops through an outlet, wherein because of the adhesion forces of the material a high pressure in the range of up to 200 MPa and generally also high melting temperatures must be applied. In this case the drops should have a size of 0.01 to 1 mm3. The opening time of the pulsable closure element at the outlet of the discharge unit should preferably lie in the range of few milliseconds and the diameter of the outlet should lie in the range of few tenths of a millimetre. For this purpose, a flexure hinge respectively solid body joint is proposed there, which is operated by means of an actuator in the form of a piezoelectric element. It is already proposed there in paragraph [0031] and [0032] to configure the actuator as a force sensing element to measure and readjust the initial stressing of the flexure hinge at the operating point. However, a separate sensor system is provided for this. The aim is to influence the form of the material to be discharged in a specific and deliberate manner such as the drop form, for example. The structure of the device forming the basis for that device is known from EP 1 886 793 A1.
With that arrangement a viscous fluid under pressure can be discharged in very small droplets through a nozzle, wherein the nozzle diameter can be about 0.1 mm and the working temperatures of the fluid can reach up to 450° C. Since a dynamic seal of structural parts sliding on one another is difficult to control in these working conditions, the flexure hinge that uses the elastic material properties for opening and closure the nozzle was employed there. The discharge of drops to construct the three-dimensional object occurs at a very high frequency of about 1 to 5 kHz. Therefore, the entire discharge unit must have a relatively high inherent rigidity. On the other hand, the desired amplitude and force of the actuator in the form of a piezoelectric element are technically not freely adjustable and also determine the costs of these components. Typically, such an element can generate forces of about 100 N and strokes of about 0.1 mm. To enable the closure element in the form of the flexure hinge to be operated and at the same time withstand the high fluid pressure of up to 200 MPa, a micromechanical adjustment to the desired operating point of the discharge unit is as necessary as a determination of the respective production-related properties of the components used such as e.g. the elasticity constant of the flexure hinge and the maximum elastic working range thereof.
This is all the more necessary when the objective is to obtain a good surface of the object to be produced, since then the drop size usually lies in the range of 0.001 mm3. To construct objects in the cm3 range, strokes of 106 and more for each object are then quickly necessary. With such a number of alternating loads it is necessary to operate the closure element in a restricted elastic range in order to obtain an acceptable service life.
The stroke of the actuator amounts to about 0.03 to 0.05 mm under the existing installation conditions. Thus, the adjustment path is smaller than the sum of all the faults arising as a result of production tolerances or different thermal expansion of the individual structural parts, the micro-deformation at the nozzle needle and nozzle etc. However, the application force must always be kept constant during the course of the discharge process, so that no unwanted discharge of material occurs and the drop size can be kept constant. A very sensitive equilibrium of forces is present because of the high pressure in the material. If the application force deviates downwards, a continuous thread discharge results instead of droplets. When the application force is too high, the nozzle can no longer be opened.